docs: def cxy(self):
          """
          `SExtractor`_'s CXY ellipse parameter in units of pixel**(-2).
  
          The ellipse is defined as
  
              .. math::
                  cxx (x - \\bar{x})^2 + cxy (x - \\bar{x}) (y - \\bar{y}) +
                  cyy (y - \\bar{y})^2 = R^2
  
          where :math:`R` is a parameter which scales the ellipse (in
          units of the axes lengths).  `SExtractor`_ reports that the
          isophotal limit of a source is well represented by :math:`R
          \\approx 3`.
          """
  
          return (2. * np.cos(self.orientation) * np.sin(self.orientation) *
                  ((1. / self.semimajor_axis_sigma**2) -
                   (1. / self.semiminor_axis_sigma**2)))